1. Field of the Invention
This invention relates to a grind-machining method for machining ceramic materials into a groove shape or a concavo-convex shape or cutting them using a grinding wheel in order to produce mechanical components made of ceramics.
2. Description of the Prior Art
Ceramic materials generally have excellent mechanical properties in hardness, strength and heat-resistance or the like, and their application as mechanical structural materials is expected. However, since the ceramic materials are typical hard and brittle materials, various problems remain unsolved in the aspect of the selection of machining methods for providing necessary geometric shapes for final products, strength or fatigue life after machining.
Grind-machining by diamond wheels has gained the widest application at present as a machining method of ceramic materials. Grind-machining using the diamond wheels is an excellent machining method in the aspects of versatility of machining equipment and a machining cost. Because the ceramic materials are the hard and brittle materials as described above, however, damages such as cracks or defects remain on the machined surface, resulting in the drop of the strength, life or reliability and preventing in most cases the practical application of the machined products.
It is known, for example, that the depth of cracks introduced at the time of grinding is affected by the grain size of the diamond grains and is as great as 20 to 40 .mu.m in the case of a silicon nitride material (Yoshikawa, "FC Report", Vol. 8, No. 5, p. 148 (1990)). The order of this crack depth is believed to be a fatal defect for practical mechanical components.
It is reported that a correlationship exists between the surface roughness of the ground surface of the silicon nitride material and its flexural strength, and the surface coarseness must be limited to below 1 .mu.m so as to maintain reliability of the strength (Itoh, "The Latest Fine Ceramics Technique", edited by Kogyo Chosakai, p. 219, (1983)).
Accordingly, there is the case where the method of securing reliability of the strength must be employed by grinding the surface layer, where defects remain, by free grains, such as lapping or polishing after grinding by diamond wheels to remove any defects. However, such an additional grinding work is extremely disadvantageous economically.
From the aspect of machining efficiency, on the other hand, it is known that machining efficiency can be drastically improved by adding a machining pressure above critical value in the grinding work of ceramic materials (Tomimori, "FC Report", Vol. 1, No. 8, p. 5 (1983)). However, experimental evaluation made by the present inventors reveals that the critical value of the machining pressure drastically increases with the improvement in the characteristics of the ceramic materials such as the hardness, the toughness, the bending strength, etc., by the improvement in the production method, and so forth.
Generally, the increase of the machining pressure can be obtained by increasing the mechanical rigidity of machining equipment. With the increase of the critical value of the machining pressure resulting from the improvement of the characteristics of the ceramic materials, however, there is a limit to the increase of the machining rigidity, and the increase of the machining cost arises due to this increase of rigidity. Furthermore, the increase of the machining pressure causes the residual defects more likely to occur in the workpieces.
As described above, mutual dependence exists between machining efficiency and the residual defects after machining in the grinding work of the ceramic materials, so that when machining efficiency is improved, the residual defects increase and machining efficiency must be limited to a low level in order to reduce the residual defects.